Oxide superconducting wire

ABSTRACT

An oxide superconducting wire includes a superconducting laminate including an oxide superconducting layer disposed, either directly or indirectly, on a substrate, and a stabilization layer which is a Cu plating layer covering an outer periphery of the superconducting laminate. An average crystal grain size of the Cu plating layer is 3.30 μm or more and equal to or less than a thickness of the Cu plating layer.

CROSS-REFERENCE TO RELATED APPLICATION

Priority is claimed on Japanese Patent Application No. 2019-064777 filed in Japan on Mar. 28, 2019, the contents of which are incorporated herein by reference in its entirety.

TECHNICAL FIELD

The present invention relates to an oxide superconducting wire.

BACKGROUND

The RE123-based oxide superconductor (REBa₂Cu₃O_(y), RE is a rare earth element) shows superconductivity at a temperature (approximately 90K) exceeding the liquid nitrogen temperature (77K). Since such superconductors have a higher critical current density in a magnetic field than other high-temperature superconductors, they are expected to be applied to coils, power cables, and the like. For example, Patent Document 1 describes an oxide superconducting wire in which an oxide superconducting layer and an Ag stabilization layer are formed on a substrate, and then a Cu stabilization layer is formed by electroplating.

[Patent Document 1] Japanese Patent Application, First Publication, No. 2007-80780

The stabilization layer serves as a detour for the current flowing at that time when the superconducting state of the oxide superconductor becomes partially unstable. Therefore, it is required that the electrical resistance of the stabilization layer is low at a low temperature.

One or more embodiments of the present invention provide an oxide superconducting wire having a stabilization layer having a low electrical resistance at a low temperature.

SUMMARY

A first aspect of the present invention is an oxide superconducting wire which includes a superconducting laminate including an oxide superconducting layer on a substrate, and a stabilization layer which is a Cu plating layer covering the outer periphery of the superconducting laminate, and an average crystal grain size of the Cu plating layer is 3.30 μm or more and equal to or less than a thickness of the Cu plating layer.

A second aspect of the present invention is an oxide superconducting wire which include a superconducting laminate including an oxide superconducting layer on a substrate, and a stabilization layer which is a Cu plating layer covering the outer periphery of the superconducting laminate, and an average number of grain boundaries per 100 μm length of the Cu plating layer is 30 or less.

A third aspect of the present invention is the oxide superconducting wire according to the first or second aspect, in which the residual resistance ratio of the stabilization layer, which is a ratio of a specific resistance at 293K to a specific resistance at 15K, is 110 or more and 130 or less.

According to one or more embodiments of the present invention, an average crystal grain size of the Cu plating layer constituting the stabilization layer is large, or the average number of grain boundaries per unit length is small, so that a stabilization layer having the low electrical resistance at a low temperature can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of an oxide superconducting wire according to one or more embodiments.

DETAILED DESCRIPTION

Hereinafter, one or more embodiments of the present invention will be described with reference to the drawings.

As shown in FIG. 1 , an oxide superconducting wire 10 according to one or more embodiments includes a superconducting laminate 15 and a stabilization layer 16 that covers the outer periphery of the superconducting laminate 15. The superconducting laminate 15 may include an oxide superconducting layer 13 on a substrate 11. The superconducting laminate 15 may be a structure including, for example, the substrate 11, an intermediate layer 12, an oxide superconducting layer 13, and a protection layer 14.

The substrate 11 is, for example, a tape-shaped metal substrate including a first main surface 11 a and a second main surface 11 b on both sides in a thickness direction, respectively. Specific examples of the metal constituting the metal substrate include nickel alloys represented by Hastelloy (registered trademark), stainless steel, oriented Ni—W alloys in which a texture is introduced into the nickel alloy, and the like. When an oriented substrate in which the arrangement of metal crystals is aligned and oriented is used as the substrate 11, the oxide superconducting layer 13 can be directly formed on the substrate 11 without forming the intermediate layer 12. A side on which the oxide superconducting layer 13 is formed on the substrate 11 is referred to as a first main surface 11 a, and the back surface opposite to the first main surface 11 a is referred to as a second main surface 11 b. The thickness of the substrate 11 may be adjusted as appropriate, and is, for example, in the range of 10 to 1000 μm.

The intermediate layer 12 may have a multi-layer structure, and may have a diffusion prevention layer, a bed layer, an orientation layer, a cap layer, and the like in the order from a side of the substrate 11 to a side of the oxide superconducting layer 13, for example. These layers are not always provided one by one, and a portion of the layers may be omitted, or two or more layers of the same type may be repeatedly laminated. The intermediate layer 12 may be a metal oxide. By film-forming the oxide superconducting layer 13 on the intermediate layer 12 having excellent orientation, it becomes easy to obtain the oxide superconducting layer 13 having excellent orientation.

The oxide superconducting layer 13 is composed of, for example, an oxide superconductor. Examples of the oxide superconductor include a RE-Ba—Cu—O-based oxide superconductor represented by the general formula REBa₂Cu₃O_(y) (RE123) and the like. Examples of the rare earth element RE include one of or two or more of Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. In the general formula of RE123, y is 7-x (oxygen deficiency amount). In addition, the ratio of RE:Ba:Cu is not limited to 1:2:3, and there may be an indefinite ratio. The thickness of the oxide superconducting layer 13 is, for example, approximately 0.5 to 5 μm.

Artificial pins made of different materials may be introduced into the oxide superconducting layer 13 as artificial crystal defects. Examples of different materials used for introducing artificial pins into the oxide superconducting layer 13 can include at least one or more kinds of BaSnO₃(BSO), BaZrO₃ (BZO), BaHfO₃ (BHO), BaTIO₃ (BTO), SnO₂, TiO₂, ZrO₂, LaMnO₃, ZnO, and the like.

The protection layer 14 has functions such as bypassing overcurrent and suppressing chemical reaction that occurs between the oxide superconducting layer 13 and the layer provided on the protection layer 14. Examples of the material constituting the protection layer 14 can include silver (Ag), copper (Cu), gold (Au), or alloys including one or more kinds of these (for example, Ag alloy, Cu alloy, Au alloy). The thickness of the protection layer 14 may be approximately 1 to 30 μm, and when the protection layer 14 is thinned, the thickness may be 10 μm or less, 5 μm or less, 2 μm or less, or the like. The protection layer 14 may also be formed on a side surface 15 s of the superconducting laminate 15 or a second main surface 11 b of the substrate 11. The thicknesses of the protection layers 14 formed on the different surfaces of the superconducting laminate 15 may be substantially the same or different. The protection layer 14 may be constituted by two or more kinds of metals or two or more metal layers. The protection layer 14 can be formed by a vapor deposition method, a sputtering method, or the like.

The stabilization layer 16 can be formed over the entire periphery including the first main surface 15 a, the second main surface 15 b, and the side surface 15 s of the superconducting laminated body 15. The first main surface 15 a of the superconducting laminate 15 is, for example, the surface of the protection layer 14; however, is not limited thereto. The second main surface 15 b of the superconducting laminate 15 is, for example, the second main surface 11 b of the substrate 11; however, the present invention is not limited thereto, and for example, the second main surface 15 b may be a surface on the protection layer 14 when the protection layer 14 is also formed on the second main surface 11 b of the substrate 11. The side surfaces 15 s of the superconducting laminate 15 are the respective surfaces on both sides in the width direction.

The stabilization layer 16 has functions such as bypassing the generated overcurrent and mechanically reinforcing the oxide superconducting layer 13 and the protection layer 14. The stabilization layer 16 is a plating layer constituted by a metal such as copper (Cu). The thickness of the stabilization layer 16 is not particularly limited; however, may be approximately 2 to 300 μm, and may be, for example, 200 μm or less, 100 μm or less, 50 μm or less, approximately 20 μm, approximately 10 μm, approximately 5 μm, or the like. The thicknesses of the stabilization layers 16 formed on the first main surface 15 a, the second main surface 15 b, and the side surfaces 15 s of the superconducting laminated body 15 may be substantially equal to each other.

As an index for evaluating the electrical resistance or conductivity of the stabilization layer 16 at a low temperature, a residual resistance ratio (RRR) can be described. The residual resistance ratio is the ratio of the specific resistance (resistivity) at two predetermined temperatures, and is obtained by RRR=ρ_(high)/ρ_(low) as the ratio of the specific resistance ρ_(high) at a high temperature and the specific resistance ρ_(low) at a low temperature. As an example, the ratio of the specific resistance ρ_(293K) at 293K and the specific resistance ρ_(15K) at 15K is ρ_(293K)/ρ_(15K). It is shown that the larger the RRR, the higher the conductivity at a low temperature than at a high temperature (normal temperature). The Cu plating layer ρ_(293K)/ρ_(15K) may be 100 or more, for example.

In the Cu plating layer constituting the stabilization layer 16, the average crystal grain size of the Cu plating layer may be 2 μm or more. Since the average crystal grain size of the Cu plating layer is large, the electrical resistance at a low temperature can be lowered. The average crystal grain size of the Cu plating layer may be 2.5 μm or more, or 3 μm or more. The upper limit of the average crystal grain size of the Cu plating layer is not particularly limited; however, examples thereof include 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, and 10 μm. The average crystal grain size of the Cu plating layer is generally equal to or less than the thickness of the Cu plating layer.

In addition, in the Cu plating layer constituting the stabilization layer 16, the average number of grain boundaries per 100 μm length of the Cu plating layer may be 50 or less. Since the average number of grain boundaries per unit length of the Cu plating layer is small, the electrical resistance at a low temperature can be lowered. Examples of the unit length include a length of 100 μm in the longitudinal direction of the oxide superconducting wire 10. The average number of grain boundaries per 100 μm length of the Cu plating layer may be 40 or less, or 35 or less. The lower limit of the average number of grain boundaries per 100 μm length of the Cu plating layer is not particularly limited; however, examples thereof include 10, 15, 20, 25, 30 grains, or the like.

The average crystal grain size of the Cu plating layer and the average number of grain boundaries per unit length of the Cu plating layer can be measured by, for example, a cross-sectional photograph of the Cu plating layer using a scanning electron microscope (SEM).

The copper plating layer constituting the stabilization layer 16 can be formed by, for example, electroplating. When the copper plating layer is formed by electroplating, a metal layer such as silver (Ag), copper (Cu), tin (Sn) may be formed in advance as a base layer by a vapor deposition method, a sputtering method, or the like. Examples of the plating bath used for electroplating the Cu plating layer can include a copper sulfate plating bath, a copper cyanide plating bath, and a copper pyrophosphate plating bath. As the copper sulfate plating solution, an aqueous solution including copper sulfate pentahydrate, sulfuric acid, additives, chlorine ions and the like is generally used.

At least a portion of the Cu plating layer can be formed by electroless plating. In such a case, a formaldehyde bath, a glyoxylic acid bath, a hypophosphate bath, a cobalt salt bath, and the like are used. A general formaldehyde bath uses a plating solution including a cupric salt, a reducing agent (formaldehyde, and the like), a complexing agent (Rossel salt, and the like), a pH adjuster (sodium hydroxide), and an additive (cyanide).

As a method of adjusting the average crystal grain size of the Cu plating layer or the average number of grain boundaries per unit length of the Cu plating layer, changing the conditions in the electroplating of Cu by at least one or more can be described. Specific conditions for electroplating can include, for example, the concentration of the plating solution, the type of plating bath, the current density, the degree of overvoltage, the temperature, the presence or absence of additives, the presence or absence of heat treatment after electroplating, or the like. For example, the higher the current density, the smaller the average crystal grain size of the Cu plating layer tends to be. In addition, by performing the heat treatment after electroplating, the average crystal grain size of the Cu plating layer becomes large. The additive for the plating bath is not particularly limited; however, examples thereof can include a complexing material, a pH adjuster, a leveler, and the like.

According to the oxide superconducting wire 10 of one or more embodiments, in the Cu-plated layer constituting the stabilization layer 16, the crystal grain size is large or the average number of grain boundaries per unit length is small, electrical resistance due to the grain boundaries can be suppressed. The electrical resistance of a metal is divided into a portion having the large temperature dependence due to such as thermal vibration of a metal atom and a portion having the small temperature dependence (residual resistance) due to the imperfections of the metal crystal. The electrical resistance of the portion with the large temperature dependence decreases at a low temperature; however, the residual resistance remains as a finite value even at a low temperature. Therefore, by reducing the residual resistance, the RRR can be increased and the conductivity at a low temperature can be increased.

Although one or more embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the gist of the present invention. Modifications include addition, replacement, omission, and other changes of components in each embodiment.

The film forming method of the intermediate layer 12 and the oxide superconducting layer 13 is not particularly limited as long as an appropriate film forming can be performed according to the composition of the metal oxide. Examples of the film forming method include a sputtering method, a dry film forming method such as a vapor deposition method, and a wet film forming method such as a sol-gel method. The vapor deposition methods include an electron beam deposition (IBAD) method, a pulsed laser deposition (PLD) method, and chemical vapor deposition (CVD) method.

The diffusion prevention layer of the intermediate layer 12 has a function of suppressing a portion of the components of the substrate 11 from diffusing and being mixed as impurities to the oxide superconducting layer 13 side. Examples of the diffusion prevention layer include Si₃N₄, Al₂O₃, GZO(Gd₂Zr₂O₇), and the like. The thickness of the diffusion prevention layer is, for example, 10 to 400 nm.

The bed layer of the intermediate layer 12 has functions such as reducing the reaction at the interface between the substrate 11 and the oxide superconducting layer 13 and improving the orientation of the layer formed on the substrate 11. Examples of the material of the bed layer include Y₂O₃, Er₂O₃, CeO₂, Dy₂O₃, Eu₂O₃, Ho₂O₃, La₂O₃, and the like. The thickness of the bed layer is, for example, 10 to 100 nm.

The orientation layer of the intermediate layer 12 is formed from a biaxially oriented substance to control the crystal orientation of the cap layer formed thereon. Examples of the material of the orientation layer include metal oxides such as Gd₂Zr₂O₇, MgO, ZrO₂—Y₂O₃(YSZ), SrTiO₃, CeO₂, Y₂O₃, Al₂O₃, Gd₂O₃, Zr₂O₃, Ho₂O₃, Nd₂O₃, and the like. The oriented layer may be formed by the IBAD method.

The cap layer of the intermediate layer 12 is formed on the surface of the orientation layer, and the crystal grains are oriented in an in-plane direction. Examples of the material of the cap layer include CeO₂, Y₂O₃, Al₂O₃, Gd₂O₃, ZrO₂, YSZ, Ho₂O₃, Nd₂O₃, LaMnO₃, and the like. The thickness of the cap layer is, for example, 50 to 5000 nm.

In order to secure electrical insulation with respect to the periphery of the oxide superconducting wire, an insulating tape such as polyimide may be wrapped or a resin layer may be formed around the outer periphery of the oxide superconducting wire. An insulating coating layer such as an insulating tape or a resin layer is not always necessary, and an insulating coating layer may be appropriately provided depending on the use of the oxide superconducting wire, or a configuration without an insulating coating layer may be provided.

To manufacture a superconducting coil using an oxide superconducting wire, for example, the oxide superconducting wire is wound along the outer peripheral surface of the winding frame with the required number of layers to form a coil-shaped multi-layer wound coil, and the oxide superconducting wire can be fixed by impregnating a resin such as an epoxy resin so as to cover the wound oxide superconducting wire.

EXAMPLES

Hereinafter, a method of manufacturing the oxide superconducting wire 10 will be described with reference to specific examples. The following examples do not limit the present invention.

First, a superconducting laminate 15 having a predetermined width was prepared by the following procedure.

(1) Polishing of the tape-shaped substrate 11 of Hastelloy (registered trademark) C-276.

(2) Degreasing and cleaning of the substrate 11 with acetone.

(3) Film formation of the Al₂O₃ diffusion prevention layer by an ion beam sputtering method.

(4) Film formation of the Y₂O₃ bed layer by an ion beam sputtering method.

(5) Film formation of the MgO oriented layer by an IBAD method.

(6) Film formation of the CeO₂ cap layer by a PLD method.

(7) Film formation of the GdBa₂Cu₃O₇-x oxide superconducting layer 13 by a PLD method.

(8) Film formation of the Ag protection layer 14 by a sputtering method from the surface direction of the oxide superconducting layer 13.

(9) Oxygen anneal treatment of the superconducting laminate 15.

(10) Thinning treatment of the superconducting laminate 15 by slitting with a width of 4 mm.

Next, a Cu base layer was formed on the superconducting laminate 15 by a sputtering method from the direction of the first main surface 15 a and the direction of the second main surface 15 b.

Next, a stabilization layer 16 having a thickness of 20 μm was formed by copper sulfate plating.

In the examples, the oxide superconducting wires 10 of sample numbers 1 to 5 were obtained under the conditions shown in Table 1.

TABLE 1 HEAT CURRENT No. PLATING SOLUTION TREATMENT DENSITY 1 COPPER SULFATE PENTAHYDRATE, SULFURIC NOT 10 A/dm² ACID, HYDROCHLORIC ACID, AND ADDITIVES PERFORMED 2 COPPER SULFATE PENTAHYDRATE, SULFURIC NOT 10 A/dm² ACID, AND HYDROCHLORIC ACID PERFORMED 3 COPPER SULFATE PENTAHYDRATE, SULFURIC NOT 10 A/dm² ACID, HYDROCHLORIC ACID, AND ADDITIVES PERFORMED 4 COPPER SULFATE PENTAHYDRATE, SULFURIC PERFORMED 10 A/dm² ACID, HYDROCHLORIC ACID, AND ADDITIVES 5 COPPER SULFATE PENTAHYDRATE, PERFORMED 10 A/dm² CHLORINE, AND LEVELER

Next, with respect to the obtained oxide superconducting wire 10, the residual resistivity ratio and the average crystal grain size of the Cu plating layer constituting the stabilization layer 16 were measured.

The residual resistivity ratio (RRR) of the Cu plating layer was calculated as ρ_(293K)/ρ_(15K) by the ratio of the specific resistance ρ_(293K) at 293K and the specific resistance ρ_(15K) at 15K.

As the average crystal grain size of the Cu plating layer, 45 cross-sectional SEM photographs (23 μm×23 μm viewing range) parallel to the longitudinal direction of the oxide superconducting wire 10 were taken for each one sample, three line segments were drawn in the longitudinal direction of the oxide superconducting wire 10 for each one photograph, the number of crystal grains that are completely cut by the line segments according to the cutting method of JIS H 0501 (copper grain size test method) was counted, and the crystal grain size in μm unit obtained as an average value of the cutting length thereof was used as it was for the average crystal grain size of the Cu plating layer.

The three line segments drawn in the longitudinal direction of the oxide superconducting wire 10 were located at a depth of approximately 3.2 μm, approximately 10.0 μm, and approximately 16.8 μm from the surface of the stabilization layer 16 having a thickness of 20 μm (i.e., positions of approximately 16%, approximately 50%, and approximately 84%, respectively, with respect to the thickness of the stabilization layer 16).

The average number of grain boundaries (pieces) per 100 μm length of the Cu plating layer was calculated as a numerical value obtained by dividing the 100 μm length by the above-described average crystal grain size (μm).

The measurement results described above are shown in Table 2.

TABLE 2 AVERAGE NUMBER OF GRAIN BOUNDARIES PER AVERAGE CRYSTAL RESIDUAL 100-μm LENGTH GRAIN SIZE RESISTANCE EVALUATION No. [PIECES] [μm] RATIO RESULT 1 170 0.59 5 NOT GOOD 2 130 0.77 45 NOT GOOD 3 120 0.83 60 NOT GOOD 4 30 3.30 110 GOOD 5 25 3.95 130 GOOD

When a tape-shaped Cu foil, which has a proven suitable for a stabilization layer for oxide superconducting wires, is used as the stabilization layer, the residual resistivity ratio is approximately 100. Therefore, as an evaluation result, a sample having a residual resistivity ratio of 100 or more was determined to be a non-defective product (Good), and a sample having a residual resistivity ratio of less than 100 was determined to be a defective product (Not Good). It was shown that the larger the average crystal grain size of the Cu plating layer, the better the value of the residual resistivity ratio of the Cu plating layer.

Although the disclosure has been described with respect to only a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that various other embodiments may be devised without departing from the scope of the present invention. Accordingly, the scope of the invention should be limited only by the attached claims.

DESCRIPTION OF THE REFERENCE SYMBOLS

10: Oxide superconducting wire, 11: Substrate, 11 a: First main surface of the substrate, 11 b: Second main surface of the substrate, 12: Intermediate layer, 13: Oxide superconducting layer, 14: Protection layer, 15: Superconducting laminate, 15 a: First main surface of superconducting laminate, 15 b: Second main surface of superconducting laminate, 15 s: Side surface of superconducting laminate, 16: Stabilization layer 

The invention claimed is:
 1. An oxide superconducting wire comprising: a superconducting laminate comprising a substrate, a protection layer, and an oxide superconducting layer disposed, either directly or indirectly, on the substrate; and a stabilization layer which is a Cu plating layer covering an outer periphery of the superconducting laminate, wherein the protection layer is a base layer of the Cu plating layer and consists essentially of Ag or Ag alloy, wherein an average crystal grain size of the Cu plating layer is 2 μm or more and equal to or less than a thickness of the Cu plating layer, and wherein the oxide superconducting layer has an orientation.
 2. An oxide superconducting wire comprising: a superconducting laminate comprising a substrate, a protection layer, and an oxide superconducting layer disposed, either directly or indirectly, on the substrate; and a stabilization layer which is a Cu plating layer covering an outer periphery of the superconducting laminate, wherein the protection layer is a base layer of the Cu plating layer and consists essentially of Ag or Ag alloy, wherein an average number of grain boundaries per 100 μm length of the Cu plating layer along a longitudinal direction of the oxide superconducting wire is 50 or less, and wherein the oxide superconducting layer has an orientation.
 3. The oxide superconducting wire according to claim 1, wherein a residual resistance ratio of the stabilization layer, which is a ratio of a specific resistance at 293K to a specific resistance at 15K, is 110 or more and 130 or less.
 4. The oxide superconducting wire according to claim 2, wherein a residual resistance ratio of the stabilization layer, which is a ratio of a specific resistance at 293K to a specific resistance at 15K, is 110 or more and 130 or less.
 5. The oxide superconducting wire according to claim 1, wherein an average crystal grain size of the Cu plating layer is 3.30 μm or more and equal to or less than a thickness of the Cu plating layer.
 6. The oxide superconducting wire according to claim 2, wherein an average crystal grain size of the Cu plating layer is 3.30 μm or more and equal to or less than a thickness of the Cu plating layer.
 7. The oxide superconducting wire according to claim 1, wherein an average crystal grain size of the Cu plating layer is 3.30 μm or more and 3.95 μm or less.
 8. The oxide superconducting wire according to claim 2, wherein an average crystal grain size of the Cu plating layer is 3.30 μm or more and 3.95 μm or less.
 9. The oxide superconducting wire according to claim 1, further comprising a Cu base layer between the superconducting laminate and the stabilization layer.
 10. The oxide superconducting wire according to claim 2, further comprising a Cu base layer between the superconducting laminate and the stabilization layer. 